This invention relates to telephone line circuits and, more particularly, to line circuits for supplying battery feed to one or more remote telephone sets.
Most line circuits for supplying the DC current required by the telephone sets' carbon microphone employ a line transformer. The transformer permits a balanced line circuit to be connected to an unbalanced switching path, it provides impedance transformation, and, if properly connected, it prevents longitudinal voltages appearing on the balanced line side from being coupled to its unbalanced side.
The line windings of the transformer either may be connected to carry the DC line current needed by the remote carbon microphone or they may be isolated from the DC current by means of a blocking capacitor. In the latter case, shunt feed inductors must usually be employed to prevent the low-impedance battery supply from short circuiting the AC components of the speech signal.
Heretofore, it has been suggested that the bulky current feed inductors could be replaced by a pair of opposite conductivity-type transistors biased for constant current operation to prevent excessive current on short loops and to provide adequate current on long loops. Examples of this type of shunt feed may be found in J. K. Livingstone U.S. Pat. No. 3,035,122, issued May 15, 1962, and in S. Orbach U.S. Pat. No. 3,955,052, issued May 4, 1976. It is to be noted, however, that these shunt feed arrangements require the use of a high quality blocking capacitor having low leakage and adequate capacitance to provide good AC coupling of speech signals to the transformer winding. In addition, the dielectric material of the blocking capacitor should be able to withstand lightning voltage surges of several hundred volts that may occur if one of the pair of carbon block protectors develops high impedance to ground with age.
On the other hand, in the conventional series feed arrangement, the two split primary windings of the line transformer respectively carry the DC microphone current to the tip and ring conductors of the telephone line. Unfortunately, the need to carry the DC current requires that more "iron" be employed in the transformer core to avoid core saturation. The increased size and weight of such construction detracts from the utility of series feed arrangements in the newer, lightweight and compact styles of construction which have come into vogue in the telephone industry with the advent of printed wiring boards and the expanding usage of integrated circuit technology.
One approach that might at first suggest itself to one concerned with reducing the effect of transformer core saturation is to provide a third winding that would be poled oppositely with respect to the tip and ring windings so as to balance out their net DC magnetization but which would be shunted by a capacitor to bypass the speech signals. The speech signals must be bypassed to prevent the third winding from also neutralizing the AC induction of the tip and ring windings. Because of transformer action, however, the third winding also exerts a cancelling effect on the speech signals since it is magnetically coupled by the changing flux in the tip and ring windings and the shunting capacitor, if it is sufficiently large to bypass the "wire components" of speech signals, exhibits a low impedance to the "induced" speech signals. Thus, the third winding acts as a highly loaded secondary of the line transformer and, accordingly, this type of construction is not in itself effective to reduce the amount of transformer "iron" required.